Make _Thread_local work across platforms

We now rewrite the binary image at runtime on Windows and XNU to change
mov %fs:0,%reg instructions to use %gs instead. There's also simpler
threading API introduced by this change and it's called _spawn() and
_join(), which has replaced most clone() usage.

libc/thread/thread.h

@@ -15,7 +15,6 @@ enum cthread_state {
   cthread_joining = 1,
   cthread_finished = 2,
   cthread_detached = 4,
-  cthread_main = 127,
 };
 
 struct cthread_descriptor_t {